Various methods for operating steering systems of vehicles, in particular steering systems with steering assistance, are known. These so-called power-assisted steering systems are used to reduce the force which is necessary to activate the steering wheel, for example when steering in a stationary state, when maneuvering or at low velocities. For this purpose, the power-assisted steering system assists the driver during steering by virtue of the fact that the force applied by the driver for the purpose of steering, for example by means of a hydraulic pump or an electric motor, is boosted.
The power-assisted steering system here generally comprises a steering handle with which the driver predefines a steering torque which is transmitted to the steering rack which is connected to the coupled wheels. In the case of a power-assisted steering system, a steering assistance unit is now provided which comprises, for example, an electric motor and which predefines a torque or a force which acts on the steering rack and is therefore superimposed on the steering torque applied by the driver.
It is known to regulate the force acting on the steering rack by the assistance unit, as a result of which, inter alia, the inertia of the assistance unit can be compensated. In this context, an expanded regulation approach for steering mechanisms with optimized feed back is also applied. The steering system exhibits the same transmission behavior here, at least in the model, as a hydraulic power-assisted steering system (HPS—Hydraulic Power Steering).
In a dynamic case, the manual forces and torsion bar torques vary. This is due to the inertia of the steering column. However, in most situations this is not perceived by the driver. However, in the case of return movement behavior the influence is marked. This results from the inertia of the steering column and the elasticity of the torsion bar. The torsion bar must firstly accelerate the steering column, in particular in the case of a so-called hands-off operation (release of the steering wheel), in order to become destressed. The assistance force is not resumed until this destressing process occurs, and the steering system can then move back. It is to be noted that purely mechanical steering mechanisms do not exhibit this time delay.
Document DE 602 18 676 T2, which is incorporated by reference herein, discloses a method for controlling a vehicle steering device, in particular a power steering device. In the described method, at least one signal which forms a display for a state of the vehicle during a steering maneuver is provided by means of a control device. The signal is analyzed in order to determine a desired operator control torque which is to be applied to a hand wheel of the vehicle. Furthermore, the inertia and the acceleration of the hand wheel are determined, the hand wheel torque being calculated from said inertia and acceleration. A column torque is determined by means of a torsion rod. The fault signal, with which the steering device is controlled, is generated by subtracting the hand torque and the steering torque from the desired operator control torque.
Document DE 100 32 113 A1, which is incorporated by reference herein, describes a steering system having a control circuit for at least one steering actuator and a control circuit for at least one steering wheel actuator. The intention is to use the presented steering system to improve the regulating quality of the torque at the steering hand wheel. For this purpose, the steering wheel actuator is connected to the steering actuator via a control circuit. The control circuitry of the steering wheel actuator has a pilot controller. This pilot controller is determined by the characteristic variables of the inertia, angular acceleration at the steering hand wheel and the transmission ratio of a transmission.
A method for operating a steering system and for providing steering assistance in a vehicle is known from document WO/2006/021487 A1, which is incorporated by reference herein. In the vehicle, a steering wheel, a steering shaft and a steering mechanism are provided, wherein a motor acting on the steering mechanism is controlled as a function of a steering torque which is measured by a steering torque sensor. It is described here that the measured steering torque is reduced by a correction variable which is formed from the inertia torque and the rotational acceleration of the steering wheel.